In recent years, optical scanners which scan optical beams of laser light or the like have been used as optical instruments, such as bar code readers, laser printers, head mounted displays, and the like, or as optical intake devices of input devices, such as infrared cameras, and the like. Optical scanners having a structure in which a mirror obtained via silicon micromachining technology is oscillated have been proposed for this type of optical scanner.
For example, JP-A-11-52278 (“JP-A” means unexamined published Japanese patent application) (referred to below as ‘Patent document 1’) discloses an optical scanner having a silicon mirror, as shown in FIG. 17. This optical scanner is manufactured using silicon micromachining technology and is formed having an overall size of several millimeters square. A supporting substrate 71 is formed as a rectangular plate having a recessed portion 71a formed in a center portion thereof. A mirror 72 which is formed from a silicon thin film is internally supported inside this recessed portion 71a. Two torsion bars 73a and 73b which are formed integrally with the mirror 72 protrude from two ends thereof. Distal end portions of these torsion bars 73a and 73b are fixed to the supporting substrate 71, and are connected respectively to pads 74a and 74b. As a result, the mirror 72 is able to be swung or oscillated between the planar direction of the mirror and a direction which is perpendicular to the mirror surface, by the twisting of the torsion bars 73a and 73b. Impurity ions are implanted at least at peripheral areas or at the surface of the mirror 72 so as to become diffused over these areas, or alternatively, these areas may be covered by aluminum or silver or by a conductive organic thin film, resulting in these areas forming an electrode portion 75 which is electrically conductive.
In contrast, fixed electrodes 77a and 77b are located respectively at positions on both sides of the recessed portion 71a on the surface of the supporting substrate 71 via an insulator 76. These fixed electrodes 77a and 77b are formed by semiconductors or conductive materials composed of organic materials, and inner side edge portions of each of these fixed electrodes 77a and 77b are placed adjacent to the electrode portion 75 which is located at the edges on each side of the mirror 72. Thus, a condenser is formed between the electrode portion 75 and the respective fixed electrodes 77a and 77b. 
If a voltage is applied between a pad 78a of the one fixed electrode 77a and the pads 74 a and 74b of the torsion bars 73a and 73b, then this voltage is applied to the mirror electrode portion 75 which is connected to the pads 74a and 74b, and electric charges having mutually opposite polarities are accumulated on the surface of the fixed electrode 77a and the mirror electrode portion 75 so as to form a condenser. Electrostatic attraction then begins to work between the fixed electrode 77a and the mirror electrode portion 75, and the mirror 72 starts to rotate. Then, after the mirror 72 has returned to its original position, by then applying voltage between the fixed electrode 77b on the opposite side and the mirror electrode portion 75, the mirror 72 is again rotated, this time in the opposite rotation direction. By performing this type of operation repeatedly, the mirror 72 makes a swinging motion (oscillation motion) by repeating a motion of rotating between the positions of maximum rotation in the anticlockwise direction and the clockwise direction.
Further, JP-A-10-197819 (referred to below as ‘Patent document 2’) describes an optical scanner in which a mirror obtained by means of silicon micromachining technology is oscillated.
As shown in FIG. 18, this optical scanner is provided with: a plate-shaped mirror 81, which is used to reflect light; a pair of rotation supporting bodies 82, which are positioned on a straight line and support the mirror 81 from each side thereof, in which the mirror 81 are connected to the pair of rotation supporting bodies 82; a frame portion 83, which surrounds the periphery of the mirror 81; and a piezoelectric element 84, which applies translational motion to the frame portion 83. In addition, this optical scanner is structured such that the center of gravity of the mirror 81 is located at a position outside the straight line connecting together the pair of rotation supporting bodies 82.
When voltage is applied to the piezoelectric element 84, the piezoelectric element 84 is made to expand and contract, so as to vibrate in the Z axial direction. This vibration is transmitted to the frame portion 83. When the mirror 81 is made to undergo relative motion relative to the driven frame portion 83 and the vibration component in the Z axial direction is transmitted to the mirror 81, because the mirror 81 has a left-right asymmetrical mass component relative to the axis formed by the X axis rotation supporting bodies 82, rotational moment is generated in the mirror 81 centered on the X axis rotation supporting bodies 82. In this manner, the translational motion which has been applied to the frame portion 83 by the piezoelectric element 84 is transformed into rotational motion centering on the X axis rotation supporting bodies 82 of the mirror 81.
Further, JP-A-10-104543 (referred to below as ‘Patent document 3’) describes an optical scanning device, as shown in FIG. 19. In this optical scanning device, beam portions 93 and 93 extend in mutually opposite directions from both sides of a movable portion (mirror) 92 in a vibrator 91, and are connected to two arm portions 94 and 94 of a fixed portion 96. Piezoelectric thin films 95 and 95 are provided respectively on the arm portions 94 and 94 of the fixed portion 96, and these piezoelectric thin films 95 and 95 are driven by the same signal which includes higher order vibration frequencies.
However, the optical scanner described in Patent document 1 is manufactured to be several millimeters square using silicon micromachining technology, and the electrode portion 75 is formed on at least peripheral areas or on the surface of the mirror 72. In addition, the pads 74a and 74b are provided on the torsion bars 73a and 73b, and it is necessary to place the respective fixed electrodes 77a and 77b and pads 74a and 74b at positions on both sides of the surface of the supporting substrate 71 via the insulator 76.
In this manner, because the electrode portion 75 is formed on at least peripheral areas or on the surface of the mirror 72, and the pads 74a and 74b are formed on the torsion bars 73a and 73b, and the respective fixed electrodes 77a and 77b and pads 78a and 78b are formed at positions on both sides of the surface of the supporting substrate 71 via the insulator 76, the manufacturing of this optical scanning device is complex, and not only have the causes for possible failures increased, but the time period required for manufacturing has also increased. Accordingly, there is a problem in that cost increases.
Further, in the optical scanner described in Patent document 2, since a structure is employed in which translational motion applied to the frame portion 83 by the piezoelectric element 84 is transformed into rotational motion centering on the X axis rotation supporting bodies 82 of the mirror 81, it is necessary to shift the center of gravity position of the mirror 81 relative to the torsion bars (82).
Moreover, the device also needs to have a certain thickness not only in the X-Y axial directions, but also in the Z axial direction, so that it is difficult for this device to be manufactured with a narrow thickness.
Further, the optical scanning device described in Patent document 3 has the drawback that a large torsion angle cannot be formed in the movable portion 92.
Namely, if a piezoelectric thin film 95 is formed in the two narrow beam portions 94 which are provided on the fixed portion 96, then the rigidity of this portion increases and vibration which is induced in the piezoelectric thin film 95 is not transmitted efficiently to the torsion bars 93 on the beam portions 94. As a result, the torsional vibration of the mirror 92 is reduced. Moreover, unless the vibration characteristics of the vibration source portion formed by the two beam portions 94 and the piezoelectric thin film 95 which is formed thereon are matched precisely, then the vibration amplitude of the torsional vibration of the mirror 92 becomes suppressed and, at the same time as that, torsion modes other than torsional vibration are superimposed thereon so that accurate laser beam scanning cannot be achieved. Furthermore, in order to increase the drive force for the mirror 92 by increasing the surface area of the piezoelectric thin film portion 95, it is necessary to increase the width of the beam portions 94. Because of this, an unnecessary two-dimensional vibration mode is generated in the same beam portion 94, so that at the same time as the vibration amplitude of the torsional vibration of the mirror 92 is restricted, a vibration mode other than the torsional vibration is superimposed thereon. As a result, there is caused a problem in that it is not possible to achieve accurate laser beam scanning. Moreover, because the width of the beams 94 is restricted to a narrow width, the formation of the top portion electrodes which are used to drive the piezoelectric thin film 95 formed on this portion is made more difficult because of the narrow width, so that problems arise such as the yield upon production being conspicuously affected.
FIG. 20 shows the same case as that of Patent document 3 (FIG. 3, as explained later), and shows a structure in which a piezoelectric film is formed on two narrow-width cantilever beam portions which support two torsion bars which protrude from a frame portion. The drive efficiency of the mirror portion scan angle was examined by a simulation calculation. The surface where Y=0 was taken as a plane of symmetry, and half of this was used as a model.
FIG. 21 shows the torsion angle of a mirror having a structure in which a piezoelectric film is formed on two narrow-width cantilever beam portions which support two torsion bars which protrude from the frame portion shown in FIG. 20. The drive voltage was set at 1 V, while the characteristics of a PZT-5A which are typical parameters were used for the electrical characteristics of the piezoelectric body constituted the piezoelectric film, while the characteristics of SUS 304 were used for the material of the scanner frame main body. The torsion angle of the mirror portion was small at only 0.63°.
As mentioned in the above Patent documents 1 to 3 each associated with the problems. Thus, in order to solve the problems in the techniques as described in Patent documents 1 to 3, the inventors have previously proposed an optical scanning device as described in International Patent Publication No. WO2008/044470 (referred to below as ‘Patent document 4’). This optical scanning device is one in which, using a simple structure, it is possible to efficiently generate torsional vibration in a mirror portion, by forming a piezoelectric actuator on a substrate having torsion bars which support a mirror portion using thin-film formation technology, such as an aerosol deposition method (which may be referred to below on occasion as ‘AD method’), a sputtering method, or a sol-gel method, and by then generating torsional vibration in the mirror portion using the vibration of the substrate.
The aerosol deposition method (AD method) is a method of forming a film or micro structure by bonding brittle ultra-fine-particles, which method comprises: browning the brittle ultra-fine-particles to place the particles on a substrate; applying a mechanical impact force or ultrasonic to the brittle ultra-fine-particles, to break the brittle ultra-fine-particles; having the brittle ultra-fine-particles bond together at broken faces caused by the breakage, thereby to form said film or micro structure high in density and mechanical strength composed of a brittle ultra-fine-particle material. As examples of the aerosol deposition method (AD method), use may be made, for example, of methods and apparatuses, as described in U.S. Pat. No. 6,531,187 B2 (U.S. 2002/0071905 A1), U.S. Pat. No. 6,827,634 B2 (U.S. 2001/0044259 A1), U.S. Pat. No. 7,153,567 B1, U.S. 2006/0201419 A1, and U.S. 2008/0241556 A1. Further, in the aerosol deposition method (AD method), in addition of a first step of the above-described applying a mechanical impact force or ultrasonic to the brittle ultra-fine-particles, a second step may be conducted to heat-treat the ultra-fine-particles at a low temperature lower than a sintering temperature of the same. As examples of such an aerosol deposition method (AD method) which includes the heat-treatment second step, use may be made, for example, of methods, as described in U.S. Pat. No. 7,276,193 B2 (U.S. 2005/0181208 A1) and U.S. Pat. No. 7,658,996 B2 (U.S. 2008/0220962 A1). The above-identified aerosol deposition methods (AD methods) and apparatuses for the AD methods as described in the above-mentioned U.S. Pat. No. 6,531,187 B2 (U.S. 2002/0071905 A1), U.S. Pat. No. 6,827,634 B2 (U.S. 2001/0044259 A1), U.S. Pat. No. 7,153,567 B1, U.S. 2006/0201419 A1, U.S. 2008/0241556 A1, U.S. Pat. No. 7,276,193 B2 (U.S. 2005/0181208 A1), and U.S. Pat. No. 7,658,996 B2 (U.S. 2008/0220962 A1), each are herein entirely incorporated by reference.
Reductions in size and cost may be achieved in a simple structure by the optical scanning device described in Patent document 4. However, it is difficult to finely adjust the torsional vibration in the mirror portion by the optical scanning device described in Patent document 4, and hence it is desired to make it possible to finely adjust the torsional vibration by a simple means.
In order to solve the above-mentioned problems, the present invention is contemplated for providing a method which can finely adjust a resonance frequency in an optical scanning device, in a readily manner and in a simple structure.